Preparation of aqueous slurries of finely divided fillers and their use for the production of papers having a high filler content and high dry strength

ABSTRACT

Process for treating aqueous slurries of finely divided fillers, an aqueous dispersion of at least one latex and an aqueous slurry of at least one starch being metered separately from one another into an aqueous slurry of at least one finely divided filler.

This application is a 371 of PCT/EP2008/058644 filed 4 Jul. 2008

DESCRIPTION

The invention relates to a process for treating aqueous slurries offinely divided fillers and their use for the production of papers havinghigh filler content and high dry strength.

In the production of filler-containing papers, the filler slurry isadded to the fiber suspension before this is passed on to the former ofthe paper machine. As a rule, a retention aid or retention aid system isadded to the filler/fiber suspension in order to retain as much filleras possible in the paper sheet. The addition of the filler to the paperenables the papermaker to achieve numerous improvements of the sheetproperties. These include properties such as opacity, whiteness, hapticproperties and printability.

If, in addition, the filler is cheaper than the fiber, the addition orincreased addition of filler can lead to a reduction in the proportionof fiber and hence to a reduction in the production costs of paper.Filler-containing paper or papers having a particularly high fillercontent can be more easily dried than papers which do not contain fillerand than papers having a lower filler content. As a consequence of this,the paper machine can be operated more rapidly and with lower steamconsumption, which both increases the productivity and reduces thecosts.

However, the addition of filler to the fiber suspension also hasdisadvantages which can only be partly compensated by the addition offurther paper assistants. For a given basis weight, there are limitswith regard to the amount of filler which can be used. The strengthproperties of the paper are usually the most important parameters whichlimit the amount of filler in the paper. Other factors too, such as thefiller retention, the drainage of the paper stock suspension and anyincreased chemical demand during retention and sizing, can play a rolehere.

The loss of strength properties of paper can be completely or partlycompensated in some cases by the use of dry and wet strength agents. Acustomary procedure is the addition of cationic starch as a dry strengthagent to the paper stock. Synthetic dry and wet strength agents, forexample based on cationic or anionic polyacrylamides, are also used. Theadded amount and the strengthening effect are, however, limited in mostcases. Equally, the compensating effect with respect to the loss ofstrength by increasing the filler and therefore also the increase infiller which is at all realizable is also limited. In addition, not allstrength properties are enhanced to the same degree and in some casesthey are insufficiently enhanced by the use of dry strength agents. Animportant example of this is the tear strength, which is influenced onlyslightly by the use of starch or synthetic dry strength agents incomparison with other strength parameters. On the other hand, theincrease in the filler content in the paper generally has a very strongadverse effect on the tear strength.

Further important properties are the thickness and the stiffness of thepaper. With the same basis weight, the increase of the filler contentleads to an increase in paper density and a decrease in the thickness ofthe paper sheet. The latter leads to a considerable decrease in thepaper stiffness. In many cases, this decrease in the paper stiffnesscannot be compensated by the use of dry strength agents alone.Frequently, additional measures, such as, for example, the reduction ofthe mechanical pressure in the press section in the calendering units,in calenders or in the dry end of the paper machine, are necessary. Thelatter completely or partly compensates the loss of thickness byincreasing the filler.

WO 03/087472 A1 discloses a process which describes the treatment offillers with a composition consisting of swollen starch particles andlatices. The latices used in this publication are water-insoluble andare present in the form of a dispersion. After separate preparation ofthis composition, it is added to the filler slurry; finally, theaddition to the fiber and the sheet formation are effected. According tothe teaching of WO 03/087472 A1, the starch particles are swollen starchparticles. Furthermore, the composition may also comprise othercoadditives, such as anionic or cationic coadditives. Water-solubleamphoteric copolymers are not disclosed in WO 03/087472 A1.

It was therefore the object of the present invention to provide furtheralternative processes for treating aqueous slurries of finely dividedfillers. The papers produced therewith should have strength propertieswhich are comparable with those of conventional papers having a lowfiller content. These strength properties include in particular the drybreaking length, the internal bonding strength and the stiffness of thepaper.

The object is achieved, according to the invention, by a process fortreating aqueous slurries of finely divided fillers, an aqueousdispersion of at least one latex and an aqueous slurry of at least onestarch being metered separately from one another to an aqueous slurry ofat least one finely divided filler.

According to the invention, the components latex and starch (swollen orunswollen) are added separated from one another to the aqueous slurry ofat least one finely divided filler. It is unimportant whether theaqueous dispersion of at least one latex or the aqueous slurry of atleast one starch is metered first into the aqueous slurry of the atleast one finely divided filler. The starch can either be metered inalready in the swollen state or subjected to the swelling processimmediately thereafter.

Preferably, however, first the aqueous dispersion of at least one latexis metered into the aqueous slurry of at least one finely dividedfiller. The addition of the aqueous slurry of at least one starch tothis filler-latex composition is then effected.

The aqueous slurries prepared by the process according to the inventioncomprise, for example from 1 to 70% by weight, preferably from 5 to 50%by weight, particularly preferably from 10 to 40% by weight, of at leastone finely divided filler. The amount of latex is, for example, from0.01 to 10% by weight, preferably from 0.1 to 5% by weight, particularlypreferably from 0.2 to 3% by weight, based on the filler. The meteredamount of the starch is, for example, from 0.1 to 10% by weight,preferably from 0.3 to 6% by weight, particularly preferably from 0.5 to3% by weight, based on the filler.

In the context of the present invention, the term latex is understood asmeaning water-insoluble homo- and copolymers which are preferably usedin the form of dispersions or emulsions.

The latex preferably comprises at least 40% by weight, preferably atleast 60% by weight, particularly preferably at least 80% by weight, ofso-called main monomers (a).

The main monomers (a) are selected from C₁-C₂₀-alkyl (meth)acrylates,vinyl esters of carboxylic acids comprising up to 20 carbon atoms, vinylaromatics having up to 20 carbon atoms, ethylenically unsaturatednitriles, vinyl halides, vinyl ethers of alcohols comprising 1 to 10carbon atoms, aliphatic hydrocarbons having 2 to 8 carbon atoms and oneor two double bonds or mixtures of these monomers.

For example, alkyl (meth)acrylates having a C₁-C₁₀-alkyl radical, suchas methyl methacrylate, methyl acrylate, n-butyl acrylate, ethylacrylate and 2-ethylhexyl acrylate, may be mentioned.

In particular, mixtures of the alkyl (meth)acrylate are also suitable.

Vinyl esters of carboxylic acids having 1 to 20 carbon atoms are, forexample, vinyl laurate, vinyl stearate, vinyl propionate, vinylversatate and vinyl acetate.

Suitable vinylaromatic compounds are vinyltoluene, α- andp-methylstyrene, α-butylstyrene, 4-n-butylstyrene, 4-n-decylstyrene andpreferably styrene. Examples of nitriles are acrylonitrile andmethacrylonitrile.

The vinyl halides are ethylenically unsaturated compounds substituted bychlorine, fluorine or bromine, preferably vinyl chloride and vinylidenechloride.

For example, vinyl methyl ether or vinyl isobutyl ether may be mentionedas vinyl ethers. Vinyl ethers of alcohols comprising 1 to 4 carbon atomsare preferred.

Ethylene, propylene, butadiene, isoprene and chloroprene may bementioned as aliphatic hydrocarbons having 2 to 8 carbon atoms and oneor two olefinic double bonds.

Preferred main monomers (a) are C₁-C₂₀-alkyl (meth)acrylates andmixtures of the alkyl (meth)acrylates with vinylaromatics, in particularstyrene (was also summarized as polyacrylate latex) or hydrocarbonshaving 2 double bonds, in particular butadiene, or mixtures of suchhydrocarbons with vinylaromatics, in particular styrene (also summarizedas polybutadiene latex).

In the case of polyacrylate latices, the weight ratio ofalkyl(meth)acrylates to vinylaromatics (in particular styrene) may be,for example, from 10:90 to 90:10, preferably from 20:80 to 80:20.

In the case of polybutadiene latices, the weight ratio of butadiene tovinylaromatics (in particular styrene) may be, for example, from 10:90to 90:10, preferably from 20:80 to 80:20.

In addition to the main monomers (a), the latex may comprise furthermonomers (b), for example monomers having carboxyl, sulfo or phosphonicacid groups. Carboxyl groups are preferred. For example, acrylic acid,methacrylic acid, itaconic acid, maleic acid or fumaric acid andaconitic acid may be mentioned. The content of ethylenically unsaturatedacids in the latex is in general less than 10% by weight.

Further monomers (b) are, for example, monomers comprising hydroxylgroups, in particular C₁-C₁₀-hydroxyalkyl (meth)acrylates, or amides,such as (meth)acrylamide.

Further monomers (b) are compounds which have at least two double bondscapable of free radical polymerization, preferably 2 to 6, particularlypreferably 2 to 4, very particularly preferably 2 or 3 and in particular2. Such compounds are also referred to as crosslinking agents.

The at least two double bonds of the crosslinking agents (h) which arecapable of free radical polymerization may be selected from the groupconsisting of (meth)acryloyl, vinyl ether, vinyl ester, allyl ether andallyl ester groups. Examples of crosslinking agents (b) are1,2-ethanediol di(meth)acrylate, 1,3-propanediol di(meth)acrylate,1,2-propanediol di(meth)acrylate, 1,4-butanediol di(meth)acrylate,1,6-hexanediol di(meth)acrylate, neopentylglycol di(meth)acrylate,trimethylolpropanetriol di(meth)acrylate, pentaerythrityltetra(meth)acrylate, 1,4-butanediol divinyl ether, 1,6-hexanedioldivinyl ether, 1,4-cyclohexanediol divinyl ether, divinylbenzene, allylacrylate, allyl methacrylate, methallyl acrylate, methallylmethacrylate, but-3-en-2-yl (meth)acrylate, but-2-en-1-yl(meth)acrylate, 3-methylbut-2-en-1-yl (meth)acrylate, esters of(meth)acrylic acid with geraniol, citronellol, cinnamic alcohol,glyceryl mono- or diallyl ether, trimethylolpropane mono- or diallylether, ethylene glycol monoallyl ether, diethylene glycol monoallylether, propylene glycol monoallyl ether, dipropylene glycol monoallylether, 1,3-propanediol monoallyl ether, 1,4-butanediol monoallyl etherand furthermore diallyl itaconate. Allyl acrylate, divinylbenzene,1,4-butanediol diacrylate and 1,6-hexanediol diacrylate are preferred.

Preferably used polyacrylate latices are those which are composed ofvinyl aromatics, alkyl (meth)acrylates and furthermore hydrophilicmonomers, such as, for example, (meth)acrylonitrile, (meth)acrylamideand (meth)acrylic acid. For example, such preferred polyacrylate laticescomprise 20-50% by weight of styrene, 30-80% by weight of alkyl(meth)acrylates and 0-30% by weight of further hydrophilic monomers,such as, for example, (meth)acrylonitrile, (meth)acrylamide and(meth)acrylic acid.

The latices are prepared as a rule by emulsion polymerization and thepolymer is therefore an emulsion polymer. The preparation of aqueouspolymer dispersions by the free radical emulsion polymerization processis known per se (cf. Houben-Weyl, Methoden der organischen Chemie,volume XIV, Makromolekulare Stoffe, loc. cit., page 133 et seq.).

In the emulsion polymerization for the preparation of the latices, ionicand/or nonionic emulsifiers and/or protective colloids or stabilizersare used as surface-active compounds. The surface-active substance isusually used in amounts of from 0.1 to 10% by weight, in particular from0.2 to 3% by weight, based on the monomers to be polymerized.

Customary emulsifiers are, for example, ammonium or alkali metal saltsof higher fatty alcohol sulfates, such as sodium n-laurylsulfate, fattyalcohol phosphates, ethoxylated C₈- to C₁₀-alkyl phenols having a degreeof ethoxylation of from 3 to 30 and ethoxylated C₈- to C₂₅-fattyalcohols having a degree of ethoxylation of from 5 to 50. Mixtures ofnonionic and ionic emulsifiers are also conceivable. Ethoxylated and/orpropoxylated alkylphenols and/or fatty alcohols containing phosphate orsulfate groups are furthermore suitable. Further suitable emulsifiersare mentioned in Houben-Weyl, Methoden der organischen Chemie, volumeXIV, Makromolekulare Stoffe, Georg Thieme Verlag, Stuttgart, 1961, pages192 to 209.

Water-soluble initiators for the emulsion polymerization for thepreparation of the latices are, for example, ammonium and alkali metalsalts of peroxodisulfuric acid, e.g. sodium peroxodisulfate, hydrogenperoxide or organic peroxides, e.g. tert-butyl hydroperoxide.

So-called reduction-oxidation (redox) initiator systems are alsosuitable.

The amount of initiators is in general from 0.1 to 10% by weight,preferably from 0.5 to 5% by weight, based on the monomers to bepolymerized. It is also possible to use a plurality of differentinitiators in the emulsion polymerization.

Regulators may be used in the emulsion polymerization, for example inamounts of from 0 to 3 parts by weight, based on 100 parts by weight ofthe monomers to be polymerized, by means of which the molar mass isreduced. For example, compounds having a thiol group, such as tert-butylmercaptan, thioglycolic acid ethyl acrylate, mercaptoethynol,mercaptopropyltrimethoxysilane or tert-dodecyl mercaptan or regulatorswithout a thiol group, in particular, for example, terpinolene, aresuitable.

The emulsion polymerization for the preparation of the latices iseffected as a rule at from 30 to 130° C., preferably at from 50 to 100°C. The polymerization medium may consist either only of water or ofmixtures of water and liquids miscible therewith, such as methanol.Preferably, only water is used. The emulsion polymerization can becarried out both as a batch process and in the form of a feed process,including step or gradient procedure. The feed process is preferred, inwhich a part of the polymerization batch is initially taken, heated tothe polymerization temperature and prepolymerized and the remainder ofthe polymerization batch is then fed to the polymerization zonecontinuously, stepwise or with superposition of a concentrationgradient, usually over a plurality of spatially separate feeds, one ormore of which comprise the monomers in pure or in emulsified form, whilemaintaining the polymerization. In the polymerization, a polymer seedmay also be initially taken, for example for better adjustment of theparticle size.

The manner in which the initiator is added to the polymerization vesselin the course of the free radical aqueous emulsion polymerization isknown to the average person skilled in the art. It can either becompletely initially taken in the polymerization vessel or usedcontinuously or stepwise at the rate of its consumption in the course ofthe free radical aqueous emulsion polymerization. Specifically, thisdepends on the chemical nature of the initiator system as well as on thepolymerization temperature. Preferably, a part is initially taken andthe remainder is fed to the polymerization zone according to the rate ofconsumption.

For removing the residual monomers, initiator is usually also addedafter the end of the actual emulsion polymerization, i.e. after amonomer conversion of at least 95%.

In the feed process, the individual components can be added to thereactor from above, at the side or from below through the reactor base.

After the (co)polymerization the acid groups present in the latex canalso be at least partly neutralized. This can be effected, for example,with oxides, hydroxides, carbonates or bicarbonates of alkali metals oralkaline earth metals, preferably with hydroxides, with which anydesired opposite ion or a plurality of opposite ions may be associated,e.g. Li⁺, Na⁺, K⁺, Cs⁺, Mg²⁺, Ca²⁺ or Ba²⁺. Ammonia or amines arefurthermore suitable for the neutralization. Aqueous ammonium hydroxide,sodium hydroxide or potassium hydroxide solutions are preferred forneutralization.

In the emulsion polymerization, aqueous dispersions of the laticeshaving as a rule solids contents of from 15 to 75% by weight, preferablyfrom 40% to 75% by weight, are obtained.

The glass transition temperature Tg of the latices is, for example, inthe range from −30 to 100° C., preferably in the range from −5 to 70° C.and particularly preferably in the range from 0 to 40° C. (measured bythe DSC method according to DIN EN ISO 11357).

The particle size of the latices is preferably in the range from 10 to1000 nm, particularly preferably in the range from 50 to 300 nm(measured using a Malvern® Autosizer 2 C).

The aqueous dispersions of at least one latex are used according to theinvention for treating finely divided fillers. Suitable fillers are allpigments usually used in the paper industry and comprising inorganicmaterial, e.g. calcium carbonate, which can be used in the form ofground calcium carbonate (GCC), chalk, marble or precipitated calciumcarbonate (PCC), talc, kaolin, bentonite, satin white, calcium sulfate,barium sulfate or titanium dioxide. Mixtures of two or more pigments mayalso be used. The mean particle diameter is, for example, in the rangefrom 0.5 to 30 μm, preferably from 1 to 10 μm.

The aqueous slurries, prepared according to the process according to theinvention, of finely divided fillers may also comprise, in addition tothe latices, swollen starch at least one which is likewise used for thetreatment of the finely divided fillers.

Suitable types of starch are all starches which are customary in thepaper industry and may be anionic, cationic or amphoteric. The averagemolar masses M_(w) of the starches are, for example, in the range from50 000 to 150 000 000, preferably in the range from 100 000 to 100 000000, particularly preferably in the range from 200 000 to 50 000 000.The average molecular weights M_(w) of the starches can be determined bymethods known to the person skilled in the art, for example by means ofgel permeation chromatography using a multiangle scattered lightdetector.

Suitable types of starch are natural starches, such as potato, wheat,corn, rice or tapioca starch, potato starch being preferred. It is alsopossible to use chemically modified starches, such as hydroxyethyl orhydroxypropyl starches, or starches which comprise anionic groups, suchas, for example, phosphate starch, or cationic starches which comprisequaternary ammonium groups, a degree of substitution DS=0.01 to 0.2being preferred. The degree of substitution DS indicates the number ofcationic groups which are present on average per glucose unit in thestarch. Amphoteric starches which comprise both quaternary ammoniumgroups and anionic groups, such as carboxylate and/or phosphate groups,and which, if appropriate, may also be chemically modified, e.g.hydroxyalkylated or alkyl-esterified, are particularly preferred. Thestarches can be used individually but also in any desired mixtures withone another.

Starch is preferably added in the swollen state to the aqueous slurry ofat least one finely divided filler. In principle, however, it is alsopossible to meter in the starch in unswollen form and then to carry outthe swelling process.

Regardless of the type of starch used, this swollen starch is clearlydistinguishable from the completely digested starch usually used in thepaper industry. In the case of the completely digested starch usuallyused, the starch grains have completely burst, the starch being presentin the form of a molecular dispersion. In contrast, the starch in theaqueous slurries according to the invention is swollen, i.e. the starchparticles are swollen but substantially non-fragmented starch particles.The starch is swollen but has retained its granular structure. Starchparticles swollen in this manner have as a rule a size in the range from5 to 90 μm, preferably from 30 to 70 μm, depending on the type of starchused.

Swollen starch is obtained by treatment of an aqueous compositioncomprising unswollen starch with hot water. This treatment is effectedbelow the gelatinization temperature relevant for the respective typesof starch, so that it is ensured that the starch particles only swelland do not burst. The temperature of the added hot water and theresidence time of the starch grains in the hot environment are dependenton the type of starch used; as a rule, however, the hot water hastemperatures in the range from 50 to 85° C., preferably in the rangefrom 60 to 80° C. and particularly preferably in the range from 70 to75° C.

The swelling process is stopped after a certain time, which must bedetermined depending on the type of starch used and the temperature ofthe hot water, preferably by adding cold water to the warm aqueousstarch mixture.

The swelling of the starch is described in WO 03/087472 A1, which ishereby incorporated by reference.

Furthermore, the aqueous slurries of finely divided fillers may alsocomprise up to 5% by weight, preferably up to 1% by weight, particularlypreferably from 0.01 to 0.3% by weight, of coadditives. These arepreferably added last to the aqueous slurry of the at least one finelydivided filler, i.e. after the addition of the aqueous dispersion of theat least one latex and after the addition of the aqueous slurry of atleast one starch.

In the context of the present invention, coadditives are understood asmeaning both anionic and cationic coadditives. Anionic coadditives are,for example, carboxymethylcelullose, polyacrylic acid, anionicpolyacrylamide, alginate and inorganic components, such as colloidalsilica and bentonite. Suitable cationic coadditives are, for example,chitosan, polyvinylamine, polyethyleneimine, polydiallyldimethylammoniumchloride, alaun, polyalumnum chloride and trivalent and tetravalentcations.

As described above, the treatment according to the invention of thefinely divided filler with the latex, the starch (swollen or unswollen)and, if appropriate, the coadditive is effected by separate metering ofthese components into the aqueous slurry of the finely divided filler.The minimum duration of action after the addition of the firstcomponent—preferably latex or starch—is, for example, 20 seconds. Theduration of action after the addition of the individual components ispreferably in each case 30 seconds, preferably in each case 1 minute,but not longer than 30 minutes.

As described above, in a preferred embodiment of the process accordingto the invention, the aqueous dispersion of at least one latex is firstmetered into the aqueous slurry of at least one finely divided filler.The addition of the aqueous slurry of at least one starch to thisfiller-latex composition is effected thereafter.

If coadditives are added to the aqueous slurry, the addition thereof ispreferably effected at the end after the separate metering of latex andstarch.

It is possible in principle to mix the starch in the unswollen statewith aqueous slurry of the finely divided filler which, if appropriate,already comprises at least one latex and then to carry out the swellingin the presence of this aqueous slurry of the finely divided filler. Itis also possible to carry out the swelling of the starch independentlyof the other components the filler and latex and then to mix the swollenstarch with the aqueous slurry of the finely divided filler which, ifappropriate, already comprises at least one latex. As described above,the starch is preferably added already in the swollen state to theaqueous slurry of at least one finely divided filler.

In all process variants, the preparation of an aqueous slurry of finelydivided filler is first effected, into which slurry the other twocomponents—latex and starch (swollen or unswollen)—are metered insuccession. The fillers are processed, for example, by introduction intowater to give an aqueous slurry. Precipitated calcium carbonate isusually suspended in water in the absence of dispersants. In order toprepare aqueous slurries of the other fillers, as a rule an anionicdispersant, e.g. polyacrylic acid having an average molar mass M_(w) of,for example, from 1 000 to 40 000 dalton, is used. If an anionicdispersant is used, for example, from 0.01 to 0.5% by weight, preferablyfrom 0.2 to 0.3% by weight, thereof is employed for the preparation ofaqueous filler slurries. The finely divided fillers dispersed in waterin the presence of anionic dispersants are anionic. The aqueous slurriescomprise, for example, from 10 to 30% by weight, in general 15-25% byweight, of at least one filler.

In a preferred embodiment of the process according to the invention, thestability of the latex dispersion is first reduced, independently of thesequence of addition of the individual components. In general, areduction in the stability of the latex dispersion may be advantageous,with the result that better affinity of the latex to the pigment surfaceis achieved. For example, the reduction of the stability of the latexdispersion can be achieved by:

-   -   a) heating the aqueous dispersion of at least one latex to        70° C. before the addition,    -   b) change of pH,    -   c) addition of inorganic ions having an opposite charge to the        latex dispersion, in particular addition of ions such as Ca²⁺ or        Al³⁺,    -   d) addition of multiply charged organic compounds which have an        opposite charge to the latex dispersion,    -   e) addition of polyelectrolytes which have an opposite charge to        the latex dispersion,    -   f) addition of organic solvents, such as, for example, acetone,        or    -   g) addition of hydrophobic opposite ions, such as, for example,        tetraalkylammonium ions.

The treatment of the aqueous slurry of finely divided fillers with thelatices and, the starch can be carried out continuously or batchwise,independently of the sequence of addition of the aqueous slurries ordispersions of the components—filler, latex and starch. On combinationof aqueous slurries of finely divided fillers, aqueous dispersions oflatices and aqueous slurries of starch, the filler particles are atleast partly coated or impregnated with the latices and the starch.

The mixing of the components is effected, for example, in a shear field.In general, it is sufficient if the components are stirred or aretreated in a shear field of Ultraturrax apparatus after combination. Thecombination and mixing of the constituents of the aqueous slurries canbe effected, for example, in the temperature range from 0° C. to 60° C.,preferably from 10 to 50° C. In general, the components are mixed at therespective room temperature up to a temperature of 40° C. The pH of theaqueous slurries of finely divided fillers, which slurries have beentreated with latices, starch, and, if appropriate, coadditives, is, forexample, from 5 to 11, preferably from 6 to 9, the pH of slurriescomprising calcium carbonate preferably being more than 6.5.

The invention furthermore relates to the use of the aqueous slurriesprepared according to the process of the invention as an additive to thepaper stock in the production of filler-containing paper,filler-containing cardboard or filler-containing board by drainage ofthe paper stock.

Specifically, these are filler-containing papers such as, for example,wood-free uncoated printing, writing or copying paper andwood-containing uncoated papers such as, for example, recycled newsprintor SC papers for the offset or gravure printing sector. By treatment ofthe filler added to the paper with at least one latex in combinationwith at least one swollen starch, the filler content of the paper can besubstantially increased with virtually unchanged strength properties.The filler-containing papers, cardboards and boards obtained using theaqueous slurries prepared according to the process of invention havestrength properties which are comparable with those of conventionalpapers having a low solids content.

The fillers pretreated by the process described above are mixed with thefiber in order thus to form the total paper stock. In addition to thetreated fillers and fibers the total stock may also comprise otherconventional paper additives. These include, for example, sizes, such asalkylketene dimers (AKD), alkenylsuccinic anhydrides (ASA), rosin size,wet strength agents, cationic or anionic retention aids based onsynthetic polymers. Suitable retention aids are, for example, anionicmicroparticles (colloidal silica, bentonite), anionic polyacrylamides,cationic polyacrylamides, cationic starch, cationic polyethylenimine orcationic polyvinylamine. In addition, any desired combinations thereofare conceivable, such as, for example, dual systems, which consist of acationic polymer with an anionic microparticle or an anionic polymerwith a cationic microparticle.

The invention is explained in more detail with reference to thefollowing, nonlimiting examples.

The stated percentages in the examples are percentages by weight, unlessevident otherwise from the context.

EXAMPLE 1

1.8 f of a 50% strength by weight dispersion of anionic latex(Catiofast® PR 5335 X, BASF Aktiengesellschaft) were mixed with 150 g ofa 20% strength by weight aqueous slurry of precipitated calciumcarbonate (PCC) with gentle stirring. At the same time, cationic waxycorn starch having a degree of substitution DS=0.035 was suspended inwater at 25° C. to give a 20% strength by weight slurry. The starchslurry was then diluted with 400 ml of hot water (75° C.) and gentlystirred for 90 seconds. 25 ml of this dilute starch slurry were thentaken and were placed in a beaker. The total filler slurry pretreatedwith the anionic latex was then added. During the addition andthereafter, the mixture was stirred with the aid of a Heiltof stirrer at1000 revolutions per minute (rpm). The pH of the mixture was thenadjusted to 8.5.

Comparative Example 1 According to WO 03/087472 A1

A cationic waxy corn starch having a degree of substitution DS=0.035 wassuspended in water at 25° C. to give a 20% strength by weight slurry.1.8 g of a 50% strength by weight dispersion of anionic latex(Catiofast® PR 5335 X, BASF Aktiengesellschaft) were mixed with thestarch slurry with gentle stirring. The mixture of starch and latex wasthen diluted with 400 ml of hot water (75° C.) and gently stirred for 90seconds. Thereafter, 25 ml of this dilute starch-latex slurry were takenand were placed in a beaker. 150 g of a 20% strength by weight aqueousslurry of precipitated calcium carbonate (PCC) were then added. Duringthe addition and thereafter, the mixture was stirred with the aid of aHeiltof stirrer at 1000 revolutions per minute (rpm). The pH of themixture was then adjusted to 8.5.

Production of Filler-Containing Paper EXAMPLES 2-4 Comparative Examples2-7

A mixture of bleached birch sulfate and bleached pine sulfite was beatengel-free in the ratio of 70/30 at a solids concentration of 4% in alaboratory pulper until a freeness of 30-35 was reached. An opticalbrightener (Blankophor® PSG, Bayer AG) and a cationic starch (HiCat®5163 A) were then added to the beaten stock. The digestion of thecationic starch was effected as 10% strength by weight starch slurry ina jet digester at 130° C. and with a residence time of 1 minute. Themetered amount of the optical brightener was 0.5% by weight ofcommercial product, based on the solids content of the paper stocksuspension. The metered amount of the cationic starch was 0.5% by weightof starch, based on the solids content of the paper stock suspension.The pH of the stock was in the range from 7 to 8. The beaten stock wasthen diluted to a solids concentration of 0.35% by weight by addition ofwater.

In order to determine the behavior of the aqueous filler slurriesdescribed above in the production of filler-containing paper, in eachcase 500 ml of the paper stock suspension were initially taken and ineach case the slurries treated according to the examples and a cationicpolyacrylamide as a retention aid (Polymin® KE 2020, BASFAktiengesellschaft) were metered into this pulp. The metered amount ofthe retention aid was in all cases 0.01% by weight of polymer, based onthe solids content of the paper stock suspension.

Sheets with the pretreated fillers described above were then formed(examples 2-4 and comparative examples 2-4). The amount of filler usedfor this purpose was adapted so that the filler contents were about 20%,30% and 40%. In the case of the pretreated fillers, the amount of slurrywhich has to be used in order to achieve a certain target value isalways smaller than in the case of the untreated fillers.

In addition, comparative examples with untreated filler were carried out(comparative examples 5-7). For this purpose, the amount of untreatedfiller slurry which is required in order to establish a filler contentof about 20%, 30% and 40% was first determined in preliminaryexperiments. Sheets with the untreated fillers were then formed.

The paper sheets were produced in each case on a Rapid-Köthen sheetformer according to ISO 5269/2, with a sheet weight of 70 g/m², and thendried for 7 minutes at 90° C.

Testing of the Paper Sheets

After a storage time in a conditioned chamber at a constant 23° C. and50% relative humidity for 12 hours, the dry breaking length of thesheets was determined according to DIN 54540 and the internal bondingstrength according to DIN 54516 and the stiffness according to DIN53121. The results are stated in table 1. The slurries corresponding tothe comparative examples or the comparative examples with the papersheets produced therefrom are characterized by the addition (CE). Theother examples are examples according to the invention.

TABLE 1 Slurry Example or according Dry Internal comparative to exampleor Filler breaking bonding example comparative content length strengthStiffness (CE) example (CE) [%] [m] [N] [mN] 2 1 19.1 5488 351 85.2 3 129.3 4615 287 62.1 4 1 40.9 4067 231 45.2 2 (CE) 1 (CE) 21.0 5051 31178.4 3 (CE) 1 (CE) 28.9 4335 253 57.6 4 (CE) 1 (CE) 39.1 3616 199 40.1 5(CE) PCC without 19.8 4291 214 76.3 pretreatment 6 (CE) PCC without 31.23286 143 43.5 pretreatment 7 (CE) PCC without 40.1 2387 79 26.9pretreatment

1. A process for treating an aqueous slurry of finely divided fillerscomprising combining an aqueous dispersion of at least one latex with anaqueous slurry of at least one finely divided filler and then combiningan aqueous slurry of at least one swollen starch with the aqueous slurryof at least one finely divided filler.
 2. The process according to claim1, wherein the aqueous slurry comprises from 1 to 70% by weight of atleast one finely divided filler.
 3. The process according to claim 1,wherein the amount of latex is from 0.01 to 10% by weight, based on thefiller.
 4. The process according to claim 1, wherein the metered amountof starch is from 0.1 to 10% by weight, based on the filler.
 5. Theprocess according to claim 1, wherein the latex comprises at least 40%by weight of main monomers (a) which are selected from the groupconsisting of C₁-C₂₀-alkyl (meth)acrylates, vinyl esters of carboxylicacids comprising up to 20 carbon atoms, vinyl aromatics having up to 20carbon atoms, ethylenically unsaturated nitriles, vinyl halides, vinylethers of alcohols comprising 1 to 10 carbon atoms, aliphatichydrocarbons having 2 to 8 carbon atoms and one or two double bonds, andmixtures of these monomers.
 6. The process according to claim 1, whereinthe latex comprises at least 60% by weight of butadiene, at least 60% byweight of mixtures of butadiene and styrene, at least 60% by weight ofC₁-C₂₀-alkyl (meth)acrylates, or at least 60% by weight of mixtures ofC₁-C₂₀-alkyl (meth)acrylates with styrene.
 7. The process according toclaim 1, wherein the starch is cationic, anionic or amphoteric.
 8. Theprocess according to claim 1, wherein the starch is a natural starchselected from the group consisting of potato, corn, rice and tapiocastarch.
 9. The process according to claim 1, wherein the starch is achemically modified starch.